1. Field of the Invention
The invention relates in general to a power supply device and projection apparatus using the same, and more particularly to a power supply device which regulates voltage by using a voltage-regulation feedback unit or has a protection device, and projection apparatus using the same.
2. Description of the Related Art
The projection apparatus normally uses a fan for releasing heat such that the inner system has a normal operation under suitable environmental temperature. Whether the fan stably operates is related to a power supply device in the projection apparatus.
Referring to FIG. 1, a partial circuit diagram of a conventional projection apparatus is shown. A projection apparatus 100 includes a power supply device 110, a fan 120, and a projection control unit 130. The projection control unit 130 is for generating an image beam. The power supply device 110 includes a bipolar junction transistor (BJT) M1, a metal oxide semiconductor (MOS) transistor M2 and a filter 112. The filter 112 is a LC filter for instance. The transistor M1 has an emitter coupled to a ground voltage, and a collector coupled to an operational voltage Vdd, such as 13V. The MOS transistor M2 has a gate coupled to the collector of the transistor M1, a drain coupled to the operational voltage Vdd and a source coupled to the filter 112. The fan 120 is coupled to the filter 112 and is used for releasing heat of the projection control unit 130.
As shown in FIG. 1, a pulse width modulation (PWM) signal is inputted to a base of the transistor M1 and the transistor M1 performs a power amplification operation on the PWM signal according to the operational voltage Vdd to generate a square-wave voltage Vin. The square-wave voltage Vin is current-amplified via the MOS transistor M2 to generate a voltage Vc, which is inputted to the filter 112. The filter 112 filters out a alternating-current (AC) component of the voltage Vc and accordingly outputs a direct-current (DC) voltage Vout for driving the fan 120. The air flow generated by the fan 120 releases heat of the projection control unit 130 outside the projection apparatus 100 and thus cool down the projection apparatus 100.
However, the operational voltage Vdd coupled to the power supply device 110 has normally 10% variation, which causes a large variation of the DC voltage Vout. As a result, the fan 120 generates loud noise due to rotating too fast or cannot effectively release heat of the projection apparatus 100 due to rotating too slow. Besides, the filter 112 is easily affected by loading on a subordinate circuits, such as the fan 120, and consequently, ripples are generated on waveform of the DC voltage Vout, which causes an unstable operation of the fan 120.
On the other hand, if the subordinate circuit of the power supply device 110 has shortcut situation, such as shortcut of a motor coil of the fan 120, the power supply device 110 without a shortcut protection device to stop an instant large current generated due to the circuit shortcut damages its inner circuit devices.